1. Field of the Invention
The present invention generally relates to a magnetic recording medium and a magnetic storage device employing an in-plane magnetic recording method.
2. Description of the Related Art
Recently and continuing, magnetic storage devices, for example, magnetic disk drives, are widely used for storing digitized moving picture data or music data. Especially, the magnetic storage devices are being used for storing moving picture data at home to replace the conventional home video tape devices, and because of the high access speed, compactness, and large capacity, the market for the magnetic storage devices is increasing rapidly. Because of the large size of the data to be recorded, for example, the moving picture data, the magnetic storage devices are required to have storage capacity as large as possible. So far, the recording density of the magnetic storage devices has been increased by 100% every year. In order to further increase the recording density, it is indispensable to develop techniques to further increase the recording density of magnetic recording media and magnetic recording heads.
One of the methods of increasing the recording density of a magnetic recording medium is to reduce medium noise so as to improve the signal-to-medium noise ratio. The medium noise is comprised of direct-current demagnetization noise and transition noise. If a recording layer is a metal film, the transition noise is dominant in the medium noise. The transition noise depends on a magnetization transition width a, hence, if the magnetization transition width a is reduced, the transition noise decreases accordingly. It is reported that the magnetization transition width a is expressed by the following formula (refer to T. C. Arnoldussen et al., IEEE Trans. Magn., 36, (2000) pp. 92-97).a=3.25×(1.89−1.11S*)√{square root over ((Mr×d)×df/Hc)}
where, S* represents coercive force squareness of the recording layer, Mr represents the residual magnetization of the recording layer, d represents the thickness of the recording layer, df represents an effective magnetic spacing between a magnetic recording medium and a magnetic recording head, and Hc represents the coercive force of the recording layer.
From the above formula, in order to reduce the magnetization transition width a, it is just needed to increase the coercive force squareness S*, or decrease the product of Mr×d, or increase the coercive force Hc. Particularly, the increase of the coercive force squareness S* or the decrease of the product of Mr×d can be achieved by just reducing the thickness d of the recording layer. Namely, in the recording layer, while growing in the thickness direction, the crystal grains also grow in the width direction. By reducing the thickness d of the recording layer, growth of the crystal grains in the width direction can be suppressed. As a result, miniaturization of the crystal grains is improved, and the coercive force squareness S* is increased.
On the other hand, in order to increase the coercive force Hc of the recording layer, for example, when the recording layer is formed from CoCrPt-based alloys, it is effective to increase the inclusion of Pt. By increasing the coercive force Hc, the magnetization transition width a is reduced, and it is expected that this will improve the long-term stability of the residual magnetization recorded in the recording layer, in other words, improve a thermal fluctuation resistance of the recording layer.
For example, Japanese Laid-Open Patent Application No. 2001-52330 discloses a technique in this field.
However, reduction of the thickness d of the recording layer may results in decrease of the reproduction output. Further, along with reduction of the thickness d of the recording layer and miniaturization of the crystal grains, the volume occupied by a minimum recording zone magnetically formed in the recording layer decreases, and the thermal fluctuation resistance of the recording layer degrades.
In addition, when increasing the coercive force Hc of the recording layer formed from CoCrPt-based alloys by adding Pt, if Pt is over-added, a crystal structure of a CoCr phase skews, which is a parent phase of the recording layer, and this degrades crystalline properties of the recording layer, which in turn adversely increases the medium noise and degrades the thermal fluctuation resistance of the recording layer. Further, this results in, when recording data in the recording layer, an increase of the magnitude of a recording magnetic field applied to reverse the magnetization of the recording layer, and degrades recording properties such as the overwrite property. In other words, by merely increasing inclusion of Pt in the recording layer to increase the coercive force Hc, even if the coercive force Hc Pt is increased, the medium noise cannot be reduced, which is the original object, and eventually, the thermal fluctuation resistance may be lowered.